While the present invention is generally applicable to a variety of different types of magnetic-tape storage devices, embodiments are discussed, below, in the context of the linear tape-open (“LTO”) magnetic-tape storage technology developed in the late 1990's as an open-standards alternative to proprietary magnetic-tape technologies. The LTO magnetic-tape storage technology is based on ½″ magnetic tape stored in a single-reel cartridge that is inserted into, and removed from, an LTO magnetic-tape drive. LTO magnetic-tape drives feature high rates of data transfer and LTO tape cartridges provide very large storage capacities. The LTO magnetic-tape storage devices find widespread use in a variety of data-archiving and data-backup applications.
The LTO magnetic-tape storage technology has evolved through four generations of magnetic-tape formats and magnetic-tape-drive implementations, referred to as “LTO-1,” “LTO-2,” “LTO-3,” and “LTO-4.” Additional LTO-5, LTO-6, and LTO-7 generations are currently in various stages of development. LTO-1 tape cartridges hold up to 100 gigabytes (“GB”) of data, with a maximum data-transfer rate of 15 megabytes (“MB”) per second. LTO-1 magnetic-tape format provides 384 data tracks, eight pairs of read/write element pairs per tape head, and a linear data density of 4880 bits/mm. Current LTO-4 magnetic-tape storage technology provides for storage of up to 800 GB per magnetic-tape cartridge and a maximum data-transfer rate of 120 MB per second. The LTO-4 magnetic-tape format provides 896 data tracks and 16 read/write element pairs per head, and provides a linear data density of 13,520 bits/mm. Each successive LTO generation provides a significantly higher track density by using additional numbers of increasingly narrower data tracks, correspondingly narrower read/write tape-head elements. Occasionally, as was the case with LTO 3, a greater number of read/write tape-head elements per tape head are included in the tape head, to allow for simultaneous access to a greater number of data tracks and higher data transfer rate for a given tape speed.
Backward compatibility of newer-generation tape drives with older-generation magnetic tapes is an expected and commercially necessary feature of tape drives. In general, a tape drive of generation n needs to be write-compatible with at least the generation n-1 magnetic-tape format and needs to be read-compatible with the generation n-1 magnetic-tape format and generation n-2 magnetic-tape format. Over the first four LTO generations, the backward-compatibility issues have been handled primarily by developing increasing narrower read/write elements within tape heads, by increasing the number of read/write elements within tape heads, and by appropriate modifications to internal magnetic-tape-drive components, including the servo controller, micro-controller routines, and internal signal paths. However, changes in the magnetic-tape format expected for generation LTO-6 are significant, and the changes present a significant technology challenge to producing an LTO-6 magnetic-tape drive head that is compatible with both LTO-6 and with LTO-5/4 magnetic tapes. The researchers and developers currently designing and implementing the LTO-6 generation of tape drives are therefore seeking designs and technologies to allow an LTO-6 magnetic-tape drive to read and write magnetic tapes formatted according to both the LTO-6 format and the LTO-5/4 format. Similar challenges are being addressed with regard to LTO-7 magnetic-tape drives. While these backward-compatibility issues have arisen in the context of the LTO magnetic-tape storage technology, compatibility issues similarly arise in other magnetic-tape storage technologies.